The present invention relates to a sealing device for a valve. The sealing device has two functions, which are, first in the normal state (e.g. no fire present), it completely shuts off or allows the passage of a fluid through the piping by means of a seat packing made of synthetic resinous material which has perfect and easy fluid stopping property and second, in the abnormal state where the seat packing has been broken owing to a fire, to hold the leakage of the fluid to the minimum by means of a fire-safe metal-to-metal contact seal, thereby preventing the spreading of the fire.